Category Archives: Uncertainty

Ice Arches and Gothic Cathedrals

Soaring towards heaven awash in light, Gothic Cathedrals awed medieval kings, jesters, and peasants alike. Their upward pointing arches allowed walls of stained windows to filter light into these massive buildings when most dwellings from royal castle to decrepit hut were dark, damp, and filthy. While the power of god was both invoked and abused, it was physics and engineering that allowed these cathedrals to scrape the skies. A delicate balance of forces is of the essence to avoid accelerations and collapse.

Arched windows within an arch inside the Cathedral of Reims, France.

Hence it should not surprise that ice arches buttressed by land show similar elegance and stability, but also dramatic collapse. When these ice arches form and collapse is one factor to determine when the Arctic Ocean will be free of ice in summer.

June-10, 2012 ice arch in Nares Strait between northern Greenland and Canada. The arch has been in place since Dec.-8, 2011.

Nares Strait Jun.-10, 2012 image showing land-fast ice between northern Greenland and Canada as well as the ice arch in the south (bottom left) separating sea ice from open water (North Water).

The Nares Strait ice arch forms between December and April most winters. Unlike the medieval cathedrals it consists of blocks of ice. Once in place, the arch shuts down all ice movement. The ocean water under the ice moves undisturbed southward sweeping newly formed ice away. This creates the North-Water polynya, first reported by William Baffin in his ship logs in 1616. The North Water supports wild life for millenia providing food and trading items for people. Even viking remnants from the time the first Gothic Cathedrals were built in Europe were found here: sections of chain mail, iron point blades, cloth, and boat rivets.

I want the ice arch in Nares Strait to collapse as soon as possible so that a Canadian ice breaker can get us to where we like to recover instruments and data that we deployed in 2009. And while I researched the stability of ice arches and studied Moira Dunbar’s 1969 satellite imagery, I came across a wonderful NOVA broadcast on medieval skyscrapers of glass and stone.” PBS stations will show it on Sept.-9, 2012.

Digging a little deeper, I also found a series of Open University podcasts and videos. My favorite 3-minute segment covers lines of thrust where barely connected irregular blocks of wood form a surprisingly stable yet wobbly arching bridge. If you want to build your own arch, then play interactively for fun with the physics of stone arches.

Since I want to understand and predict when the ice arch of Nares Strait collapses, I must understand how medieval architects and engineers designed their Gothic Cathedrals. I will also need understand why some cathedrals are still standing while others collapsed. My icy building blocks in Nares Strait are not as solid as the stones of Reims Cathedral, but unlike the medieval scientists, today we have computers and mathematics to help … as well as more than 800 more years of experience in science and engineering.

Ocean Warming off Greenland near Petermann Glacier

Testifying before the Select Committee on Energy Independence and Global Warming last year, I fumbled one question asked by the Honorable Chairman Edward J. Markey (D-MA): “Is it warming in the Petermann Glacier area?” I was unsure how the regionally relevant ocean temperatures had changed and how it impacts the melting glacier. A year late, we got the answer.

Floating ice shelf of Petermann Glacier on July 22, 2010 (NASA).

I was thinking of my former student Ms. Zweng. Three years earlier she had published a thorough analysis of ocean temperatures in Baffin Bay, that showed statistically significant warming by 0.11 +/- 0.06 degrees centigrade per decade for the 1916 through 2003 period (Zweng and Muenchow, 2006). But Baffin Bay is more than 800 miles away and it is not clear if those waters actually can make it to Petermann Fjord. I was also thinking of data in hand from only 80 miles away in Nares Strait whose waters definitely make it into Petermann, but I had not yet done the analyzes and thus did not know what the data would tell me. Now I do, and the peer-reviewed results (Muenchow et al., 2011) were published last week in Oceanography.

Time series of temperature (bottom) salinity (top) from the bottom of the ocean in Nares Strait between northern Greenland and Canada (from Muenchow et al, 2011). Trends are indicated for the 2003-06 and 2007-09 periods.

The data come from thermometers taking readings for years every 15 minutes. We placed the instruments on the bottom of the 300 meter deep ocean in 2003, recovered them in 2006, threw them back into the ocean in 2007 and found them again in 2009. We got data from three such instruments in 2003-06 and five in 2007-09 that all pretty much show the same thing: Bottom temperature change little during the 2003-06 period and about 0.06 +/- 0.02 degrees centigrade per year during the 2007-09 period of oberservations. Putting this together, we find a warming of 0.023 +/- 0.015 degrees centigrade per year. Next question would be, does this observed ocean warming in Nares Strait matter with regard to Petermann Glacier?

My current answer is a strong no. First, there is so much ocean heat already inside Petermann Fjord to melt away the entire floating section of the glacier (Johnson et al., 2011), that the extra ocean warming in recent years makes little difference. Second, the trends are from very short data sets that are dominated by physics unrelated to warming or could relate to a sequence of a few strong events that could either relate to man-made global warming or natural fluctuation at longer decadal cycles. This detection of signals in noise is a common problem in both engineering and geophysics, it is a required class for all our graduate students.

Very closely related is a paper entitled “Separating Signal and Noise in Atmospheric Temperature Changes: The Importance of Timescale” by Santer et al. (2011). Elegantly and comprehensively the authors expose and quantify the challenges one faces trying to extract the man-made warming signal from globally averaged near surface air temperature records sensed both from satellites and simulated in a number of numerical models. For this variable, the authors conclude convincingly, one needs records between 15-20 years long to extract a statistically significant man-made global warming signal from the much larger noise of natural variability.

So, if I had done my homework better last year, this should have been my answer to the question if it is warming in the Petermann Glacier area: “Yes, both the ocean and the atmosphere are warming in the Petermann region, but this may have little or no impact on the changing Petermann Glacier. Today we do not even know why Petermann Glacier has a floating ice shelf. Since we do not yet understand the physics of ice-ocean interactions, we can neither know nor predict what changes it has in store for us.”

Uncertainty in the Physics and Philosophy of Climate Change

I wrote this post last year for the National Journal, but it also relates to the way I think about Petermann Glacier’s ice islands. There are now at least 4 larger ice islands that formed from last year’s single calving: one is the tourist attraction off Labrador and Newfoundland, a second has left Petermann Fjord last week, a third was grounded off Ellesmere Island for much of the year and is now where #1 was Nov.-2010, while the fourth … I do not know. Last I heart, it was grounded off central Baffin Island. With this much variation of where pieces of the ice island went, how can we possibly claim any skill in predicting anything?

Neither climate nor weather is linear, but this neither makes them unpredictable nor chaotic. The simple harmonic pendulum is the essence of a linear system with clear cause and effect relations. Oscillations are predictable as long as the initial forcing is small. Furthermore, a linear trend will show the pendulum to slow down due to friction. Corrections are straightforward.

Unfortunately, climate is not a simple, harmonic, or linear system. While this does not make it unpredictable or chaotic, it means that our “common sense” and loose talk of “totality of events” can easily fool us. We know that CO2 emissions for the last 150 years changed global temperatures. We also know that our current climate system has been very stable over the last 10,000 years. What we do not yet know is how small or how large a perturbations the last 150 years have been. If the pendulum is forced too much, if the spring is stretched too far, the system will find another stable state by breaking. Climate dynamics can find an adjustment less tuned to the areas where people presently live. This is what “tipping points” are about. Only numerical experimentation with the best physics and models will suggest how close to a different stable climate state we are. The IPCC process is one way to do so.

Ice cores from Greenland contain air bubbles 100,000 years old, which clearly demonstrate that our present climate state is the “anomaly of quiet” in terms of temperature fluctuations. The absence of large fluctuations for about 10,000 years made agriculture and advanced civilizations possible. The ice cores show that abrupt climate change has happened and may happen again, not this election cycle, but it is one possibility perhaps as likely as the possibility that climate change is mundane, linear, and follows trends that we can easily correct or mitigate later. Both are excellent hypotheses.

For scientists, these are exciting times as we conduct a massive, global experiment to see how much CO2 we can add to the atmosphere to perhaps find a different climate state. Dr. Terry Joyce, Senior Scientist at Woods Hole Oceanographic Institution once said: “I’m in the dark as to how close to an edge or transition to a new ocean and climate regime we might be. But I know which way we are walking. We are walking toward the cliff.” I agree with this sentiment, but add that we do not know if this cliff is a 1000 feet fall or a 2 feet step. Can we affort to wait until we know for sure? As a scientist I do not care. As a citizen, however, I think the time to act responsibly is now.